1. Field of the Invention
The present invention relates to a sheet resist apparatus, which can be utilized in an image forming apparatus such as a copying machine, a laser beam printer, a thermal or ink-jet head printer and so on.
2. Description of the Related Arts
The inventors of the present application know one kind of such image forming apparatuses equipped with a sheet resist apparatus, which is adapted to form a loop of the recording sheet at its tip portion such that the tip portion abuts a nip portion of a pair of resist rollers by the entering force of the loop in order to eliminate a possible inclination of the recording sheet caused during its transportation from a sheet supplying unit.
In this kind of sheet resist apparatus, a pair of transporting rollers are rotated by a driving device, such as a motor, so as to transport the recording sheet from a sheet supplying unit along a transporting path. At the downstream of the transporting rollers, there is disposed a sheet sensor for detecting the tip etc. of the transported recording sheet. The sheet sensor consists of a photo-sensor, for example.
At the downstream of the sheet sensor, there are disposed a pair of resist rollers for transporting the recording sheet in synchronization with the transfer timing of the toner image at the image forming unit, wherein the resist rollers are driven by a driving device, such as a motor.
Along the transporting path between the transporting rollers and the resist rollers, there are disposed guiding plates for guiding the recording sheet from the transporting rollers to the resist rollers, one of which is disposed at the upper side of the transported recording sheet and the other of which is disposed at the lower side of the transported recording sheet.
The upper guiding plate is made in such a shape as to offer a predetermined space above the transported recording sheet, so that the transported recording sheet is formed in a loop at its tip portion, at the upstream side of the resist rollers.
The operation of thus constructed sheet resist apparatus, is as following. Namely, when a start key of the image forming apparatus is turned on, the starting signal is inputted to a control unit of the sheet resist apparatus. Then, the control unit drives and controls the transporting rollers and the resist rollers so as to transport the recording sheet from the sheet supplying unit to the resist rollers through the transporting rollers.
When the sheet sensor detects the tip of the transported recording sheet, and is turned ON, the control unit sets the timer and stops driving the resist rollers. When the timer becomes in a state of count-out after counting a predetermined time period, the timer is reset and the transporting rollers are stopped. In this procedure, the tip of the recording sheet firstly abuts to the nip portion of the resist rollers, and a loop is formed of the recording sheet by the transporting force, and at this time, the inclination of the sheet is eliminated by the formed loop.
Then, the control unit checks if it is a timing of image-forming or not, and when it comes the timing of image-forming, the control unit drives again the transporting rollers and the resist rollers in a prescribed timing to transport the recording sheet to the image forming unit, so as to forming the image on the transported recording sheet.
However, in case that the recording sheet diversifies to a certain extent and in case that the image-forming speed of the image forming apparatus such as a copying machine etc. is increased, the above mentioned type of the sheet resist apparatus cannot always transport the recording sheet properly to the image forming unit in synchronization with the image transfer timing or cannot always eliminate the inclination of the recording sheet. This is because the loop of the sheet tends to disappear quickly as the recording sheet moves back toward the transporting rollers by its restitution force in some type of rigid recording sheet, even if the loop is once formed at the upstream of the resist rollers.
The thickness of the recording sheet used in the image forming apparatus is various i.e. in a range between 52 g/m.sup.2 and 198 g/m.sup.2, for example, while various types of sheet such as a color sheet, label sheet, a sheet for OHP (Over Head Projector), etc., are utilized as the recording sheet. Accordingly, in case of a relatively rigid recording sheet, the loop formed of the recording sheet by the sheet resist apparatus disappears quickly by the restitution force of the recording sheet. If the loop formed condition of the recording sheet is kept for a long time in order to obtain the appropriate timing of image-forming, the recording sheet may be curled. If the image-forming process is performed under this curled condition, the toner image cannot be certainly transferred onto the recording sheet, with a serious drawback of causing a white void of the transferred image at the vicinity of the tip of the recording sheet.